In an apparent move to counter the expected entry by the American Telephone and
Telegraph Company into the desktop computer market, the International Business Machines
Corporation said yesterday that it would make a popular computer operating system,
originally developed by Bell Laboratories, available to users of the I.B.M. Personal
Computer.

The system - which controls essential internal operations of a computer - is
known as UNIX. It is widely used on much larger computers, primarily by engineers
and on university campuses, and can permit several computer users to work on a single
machine simultaneously, which is not now possible with the I.B.M. Personal Computer
line.

Industry experts said last night that the announcement, which came much sooner
than expected, effectively upstaged A.T.& T.'s expected introduction of its own
line of desktop computers. For some time A.T.& T., which under the terms of its
divestiture agreement is free to enter the computer business, has been expected
to incorporate UNIX in that line, making the telephone company's system particularly
attractive to users of minicomputers and mainframes that use the UNIX system.

Now I.B.M. has assured that its microcomputer line can also communicate with
larger computers that use UNIX. ''What they have done is made sure that no one else
is the first to exploit UNIX'' on a microcomputer, said David L. R. Stein, executive
vice president of the Gartner Group, a Connecticut consulting group that follows
I.B.M. closely.

I.B.M. officials declined to comment on their competition with A.T.& T., which
owns Bell Laboratories and has licensed UNIX widely, including to I.B.M. But Nadine
Fletcher, an I.B.M. spokesman, said: ''We see a major business opportunity and a
chance to offer much more to PC users.''

I.B.M.'s announcement came early last evening, and no A.T.& T. spokesman could
be reached for comment.

For users of the popular I.B.M. Personal Computer, the decision announced yesterday
makes available a tremendous amount of applications software, or programs that perform
a specific task, like determining stress factors in a building's structure, that
have been written to run under the UNIX program. Until now, those programs could
not operate on the I.B.M. Personal Computer and its more advanced cousin, the XT,
which use an operating system called PC-DOS, written by the Microsoft Corporation.

In its announcement, I.B.M. said that its version of UNIX was developed by the
Interactive Systems Corporation, and would be known as the Personal Computer Interactive
Executive. It said the system would be available in April, for a one-time charge
of $900.

The introduction of UNIX could foreshadow a new, more powerful version of the
Personal Computer that would enable several people to work on the machine at one
time, each at separate work stations. Rumors within the industry have speculated
that such a system, code- named Popcorn, would be introduced later this year as
part of a broad effort by I.B.M. to redesign its whole line of computers toward
systems compatible with the Personal Computer.